An important problem currently facing the Navy is the protection of their ships from missile attack. At present, surface ships are vulnerable to infrared seeking missiles which home-in on a large, bright infrared source such as a ship. It is desirable that these ships be able to defeat a missile attack by means such as decoying an approaching missile so that it misses its target.
A simple defensive technique against infrared seeking missiles is to deploy a screening cloud of dust or smoke to obscure the ship. However, smoke screens take a long time to deploy, and thus their use requires a relatively early warning. Smoke screens also require a large quantity of aerosol material. Screening clouds have the further disadvantage of blinding the ship as well as the attacking missile.
Rapidly deployable infrared decoys would make target acquisition more difficult for the infrared seeking missile. There are essential requirements that infrared decoys must fulfill in order to be effective. First, they must be as bright as the largest ships are in the wavelengths detected by the missile (approximately 10 kw/steradian)--existing missile infrared detectors measure the 3 to 5 .mu.m and 8 to 12 .mu.m bands. The decoys must be rapidly deployable to allow their use only when an incoming missile is detected. The decoy must produce its bright output for at least one minute so as to attract an infrared-seeking missile during its approach. Additionally, the decoy's infrared image must have a spectral content that resembles a blackbody with a temperature of about 20.degree. to 50.degree. C. above ambient temperatures. Ideally the decoy will also present its infrared image on an appropriate spatial scale so as to mislead not only present but also anticipated future advanced sensing systems. No existing decoy system possesses all of these features.
Currently, the Navy deploys flares to distract infrared-seeking missiles, but flares have several problems. Although flares are very bright, they emit a spectrum characteristic of a very hot blackbody (greater than 1,000.degree. C.) rather than a blackbody of about 20.degree. to 50.degree. C. above ambient temperatures. That is, unlike a blackbody with a relatively equal energy output across its spectrum, the majority of the flare's energy is output at wavelengths in a narrow band.
Sensors monitoring the relative intensities at several wavelengths can immediately recognize and disregard such a flare. While the infrared wavelengths put out by flares can be modified by altering their chemistry, it appears to be very difficult to make the flare look like a cooler blackbody over its complete spectrum. Another problem associated with flares is that their output is usually too spatially localized to appear ship-like. It remains likely that heat seeking missiles will be able to distinguish between decoy flares and ships.
Another approach being developed is to deploy an aerosol of particles that undergo chemical reaction or phase changes upon deployment. The reaction causes the aerosol to release heat for a short period of time. This approach has not been successful because the particles burn and cool down too quickly (in less than 10 seconds).
There have been attempts to use chemical or aerosol screens in order to decoy an infrared-seeking missile target or to attenuate infrared signals or laser light. For example, a camouflaging aerosol that can cause a false target effect within the infrared range is disclosed in U.S. Pat. No. 4,406,815 (Magnusson et al). Pyrotechnic devices in compositions for producing an infrared blocking screen are disclosed in U.S. Pat. Nos. 4,728,375 (Simpson), 4,719,856 (Joslin), and 4,719,857 (Spring). Finally, systems for attenuating or dissipating laser light energy are disclosed in U.S. Pat. Nos. 4,673,250 (Roberts et al), and 3,992,628 (Karney). However, none of these systems can effectively and reliably decoy incoming infrared-seeking missiles.
It is an object of this invention to decoy an infrared seeking missile with an aerosol.
It is a another object to have the aerosol have an infrared emission spectrum similar to a large blackbody 20.degree. to 50.degree. C. above ambient temperature.
It is another object of this invention to have the infrared emission from the aerosol continue for several minutes.
It is a further object of this invention to be able to control the spatial dispersion of infrared emissions from the aerosol.